Cast aluminum alloy members advantageous in reduced weight, the easiness of forming complicated shapes, low production cost, etc. are widely used as various parts. Particularly in automobiles, etc., JIS AC4B, ADC12, etc. of Al—Si—Cu—Mg-based aluminum alloys are used for cases and covers, and JIS AC4CH, ADC3, etc. of Al—Si—Mg-based aluminum alloys are used as underbody parts and road wheels. With energy saving and improved fuel efficiency required, further weight reduction and quality improvement are desired in cast aluminum alloy members constituting these parts.
The reduction of weight and thickness while keeping necessary strength has recently been achieved by optimum heat treatment and structure analysis using CAE, thereby meeting the above requirement of weight reduction. However, because of the later-described properties of the materials, there remains only little room for further weight reduction.
Though the above Al—Si—Cu—Mg-based aluminum alloys widely used for cases and covers have sufficient strength, they likely suffer reduced gas tightness due to corrosion when made very thin, because they contain Cu, an element having a large atomic weight and hindering corrosion resistance. Also, because the above Al—Si—Cu—Mg-based aluminum alloys have low ductility (fracture elongation of 2.0% or less), they are not easily used for members needing deformability, resulting in limited applications.
Because of larger ductility than that of Al—Si—Cu—Mg-based aluminum alloys, the above Al—Si—Mg-based aluminum alloys used for underbody parts, road wheels, etc. have large deformability. They also have good corrosion resistance because of substantially no Cu contained. Their 0.2% yield strength (strength index) is 100 MPa or more, on a level usable for vehicles, etc., and can be increased by a heat treatment, enabling thin cast members with reduced weight. However, because of Young's modulus of about 76 GPa, they cannot keep rigidity necessary for cast members when made thin, though they can keep strength and ductility. It is thus difficult to provide thinner cast members with reduced weight.
Conventional casting Al—Si—Mg-based alloys have densities of about 2.7 g/cm3, and specific rigidity (Young's modulus divided by density) of about 28 GPa/(g/cm3). However, higher demand is mounting on casting Al—Si—Mg-based aluminum alloys having larger specific rigidity with excellent strength and ductility, to provide thinner cases and covers, and lower-weight underbody parts and road wheels.
As an Al—Si—Mg-based aluminum alloy, JP 2008-291364 A discloses an aluminum alloy comprising 11.0-12.0% by weight of silicon, 0.7-2.0% by weight of magnesium, 0.1-1% by weight of manganese, at maximum 1% by weight of iron, at maximum 2% by weight of copper, at maximum 2% by weight of nickel, at maximum 1% by weight of chromium, at maximum 1% by weight of cobalt, at maximum 2% by weight of zinc, at maximum 0.25% by weight of titanium, 40 ppm of boron, and if necessary 80-300 ppm of strontium, the balance being aluminum (further elements and impurities introduced by production processes: at maximum 0.05% by weight each, and at maximum 0.2% by weight in total).
However, because the amount of Si increasing Young's modulus with a lower density is as small as 11.0-12.0% by weight, the aluminum alloy has small specific rigidity. Also, the inclusion of larger amounts of alloy elements having larger atomic numbers than that of Al, such as Mn, Fe, Cu, Ni, Cr, Co, Zn, etc., increases the density of the aluminum alloy, resulting in lower specific rigidity, as well as poorer ductility and corrosion resistance.
JP 2010-531388 A discloses a structural material of a Mg-containing, high-Si Al alloy; an ingot of the Al alloy produced by a semi-continuous casting method being subjected to a primary heat treatment to diffuse eutectic Si particles, and then thermally forged and heat-treated to a final shape and microstructure; the Al alloy being strengthened by a finer Al matrix, Si particles, and precipitated secondary particles; the Al alloy containing 0.2-2.0% by weight of Mg and 8-18% by weight of Si, and having a uniformly divided microstructure; the Al matrix having a equiaxial crystal structure having an average diameter of less than 6 μm; and the diffusion-distributed Si particles and other secondary particles having average diameters of less than 5 μm.
However, because this structural material is obtained through complicated steps comprising semi-continuous casting to form an ingot, and thermal forging, it is not suitable for cast articles directly obtained in desired shapes from a metal melt.
JP 2013-159834 A discloses a method for producing a resin-bonding cast aluminum alloy member, which comprises producing a cast aluminum alloy substrate containing 0.9-18% by mass of Si and 1.0-10.0% by mass of Mg by die-casting; and then etching the cast aluminum alloy substrate with an acidic aqueous etching solution of sulfuric acid and/or nitric acid at 30-80° C. for 5-15 minutes to dissolve Mg2Si crystals on a surface of this cast aluminum alloy substrate, thereby imparting fine roughness for excellent resin bondability to the cast aluminum alloy substrate surface.